//one - more flexible possibility since it's made with flexible If snippet

[[!If? &subject=`[[+introtext]]` &operator=`isnotempty` &then=`[[+introtext]]` &else=`[[+content:strip_tags:ellipsis=`500`]]` ]]

//the better one when it comes to performance (thank you Mark-H)
[[+introtext:default=`[[+content:strip_tags:ellipsis=`500`]]`]]